Well first I'll say, welcome to the forums.
Then I will add that you seem to be in the right place. It does look like you are headed for poly. As for advice, the best advice is open communication. Make sure the three of you are "on the same page" as to what and where you might like this relationship to lead. Be ready for the bumps, not just by looking and anticipating them, but by being ready to talk about and deal with them.
I wish you three luck with your endeavors.
Life is about the journey and not the destination,
so what better way to know life
than to wander all the roads and paths set before you.